US6942690B1 - Single-piece endoprosthesis with high expansion ratios and atraumatic ends - Google Patents

Abstract

A new endoluminal prosthesis for use in sealing a vascular graft to corporeal lumen provides for a flattened bulbous tail at the end of each cell of the prosthesis. The flattened bulbous tails reduce the amount of wear between the prosthesis and the softer material of the vascular grafts or corporeal lumen walls. A method of manufacturing a stent from a flat sheet of material is also included.

Description

RELATED APPLICATIONS

This Application is a continuation-in-part of application Ser. No. 09/837,353 filed Apr. 17, 2001 entitled “Single Piece Endoprosthesis with High Expansion Ratios,” which is a continuation-in-part of application Ser. No. 09/546,966 filed Apr. 11, 2000 entitled “Single Piece Thick-Walled Endoprosthesis.”

BACKGROUND OF THE INVENTION

This invention relates to medical devices for the treatment of vascular diseases generally referred to as endoluminal prostheses. A variety of such devices are available for a broad range of treatment modalities. Examples of such devices are “vascular grafts” and “stents.” Vascular grafts are typically used to treat weakened areas of vessels known as aneurysms. Stents are typically used to prop open a narrowed or stenosed vessel.

Stents and grafts may be delivered intraluminally through a narrow incision or a puncture in the patient's skin. The device may be mounted on a delivery catheter and inserted into a corporeal lumen through the skin. The device and catheter are then advanced through the various lumens to the site to be treated.

To accomplish this, stents and grafts are generally collapsible for delivery and expansible for treatment.

Vascular grafts are primarily composed of an artificial lumen which isolates the natural lumen from the flow of bodily fluids, such as blood. Grafts may incorporate attachment devices to secure the graft into the natural lumen and keep the graft expanded. Stents are typically formed of metallic wires or bars configured in a cylinder. Prior art stents for sealing a graft to corporeal lumen include narrow, sharp tails that can cause wear to the relatively soft graft. The wear is a function of the radial force of the stent, the sharpness of the stent, and the amount of relative motion between the stent and the graft.

The prior art also teaches methods of manufacturing stents from tubular shape material. Such methods require the manufacturer to remove material from radial surfaces to produce the stent pattern. Raw tubular shape material, as well as processing tubular shape material, is relatively more expensive than the cost and processing of the material in other forms.

Hence, those skilled in the art have recognized a need for providing a prosthesis which produces less wear between the prosthesis and the softer graft material. The need producing stents with non-tubular shape materials has also been recognized. The present invention fulfils these needs and others.

SUMMARY OF THE INVENTION

Briefly and in general terms, the present invention relates to an improved endoluminal prosthesis. This prosthesis may function as a stent or as a means to secure an endoluminal graft in a corporeal lumen such as an artery. The stent may include flattened bulbous tails to reduce the amount of wear between the stent and the softer graft material or corporeal lumen tissue. Stents typically are used to ensure the patency of diseased corporeal lumens by resisting collapse and occlusion. Endoluminal grafts typically are used to isolate diseased corporeal lumens from the flow of bodily fluids. The present invention also relates to a method of producing a tubular stent from a flat sheet of material.

The prosthesis incorporating the present invention is configured as a series of intermittently merging curved beams (e.g. leaf springs) formed into a cylinder. This cylindrical structure is capable of being compressed into a small diameter and expanded into a large diameter. To facilitate both compression and expansion the beams have a cross-section which is greater in the radial direction (thickness) than in the circumferential direction (width). The beams of the present invention are also generally continuously curved to reduce or minimize stress concentrations in the structure. The beams straighten during compression until they are nearly straight.

While compressed the thickness of the beams prevents overlap. In a tightly packed configuration, the curved beams straighten out, come together and generally lie flat in close proximity to each other. The beams resist overlap because the thickness of each beam requires substantial radial displacement to move over or under the adjacent beam. The compression of the prosthesis may be maximized by configuring the beams to fit together tightly in a collapsed condition.

While expanded and during expansion, the thickness of the beams and the configuration of the beams increase the strength of the prosthesis and reduce or minimize stress concentrations. Thicker beams provide for more material in the radial direction to prevent radial collapse. The curved configuration of the beams spreads the bending due to expansion throughout the entire length of the beam. This prevents one area of the beam from generating most of the bending and withstanding resultant stress concentrations.

In various preferred embodiments of the present invention, further improvements distribute stresses throughout the beams more evenly. For example, the extreme ends of adjacent beams may be connected by a loop or eyelet connector. In such an embodiment the stresses from bending due to compression of the prosthesis concentrate in the loop portion of the connector until the lower portion of the connector just adjacent to the loop portion closes on itself, bringing the adjacent beams into contact. Further compression after that point concentrates stresses in the beam below the loop. A similar result can be achieved by configuring the beams to form a significant area of contact adjacent other types of connectors prior to full compression of the prosthesis.

The present invention is a single integrated structure without welds or fasteners. This may be accomplished by removing almond-shaped cells from a thick-walled cylinder. This eliminates the need to construct the prosthesis from individual pieces and possible weak points created by fasteners or joining.

In a first embodiment, the prosthesis may have curved beams which are only merged to adjacent beams at their end points. This creates a single repetitive pattern around the circumference of the cylinder, with each beam merged to opposite adjacent beams at opposite end points. This embodiment may be viewed as the simplest structure to include the invention described herein. It includes alternating half-cells divided by curved beams. This embodiment is not necessarily short, as the beams may be of any length. However, it may be viewed as the shortest configuration for any given cell size.

In a second embodiment, the prosthesis may have curved beams like leaf springs which are repeatedly merged to alternating adjacent beams throughout their length. This second embodiment may also be viewed as the single repetitive pattern of the first embodiment repeated throughout the length of the prosthesis. For example, a prosthesis may be comprised of two or more of the single pattern prosthesis connected end to end. Instead of actually connecting the prosthesis, they may be formed as a single structure. Thereby, the beams could be viewed as continuous throughout the length of the prosthesis. The beams would then have many curved portions which bring them in connection with alternating adjacent beams at merge sections.

The prosthesis may also embody these curved beams forming individual cylindrical elements and connected together by separate elements. Thus, a variety of prosthesis may be formed by connecting different cylindrical elements together with different connecting elements. One configuration includes cylindrical elements having curved beams which are only merged to adjacent beams at their end points connected to cylindrical elements having curved beams which are repeatedly merged to alternating adjacent beams throughout their length. This provides a prosthesis having varying strength and flexibility throughout its length.

In the compressed condition the prosthesis may be intraluminally inserted and delivered within a corporeal lumen. Once delivered to the site to be treated, the prosthesis may be expanded and imbedded into the interior of the lumen. Various methods for intraluminally expanding prostheses are well-known in the art. Expansion due to spring forces is particularly suited for this invention. The super-elastic properties of Nickel—Titanium alloys (for example Nitinol) allow a great amount of expansion and compression of structures without permanent deformation. Thus a prosthesis made of such material may be compressed into a very small configuration, and will spring back into a preset form when released. Other known methods of expansion include balloon expansion, and expansion due to the highly elastic properties of certain alloys.

The present invention may also be balloon expandable. To expand the prosthesis by balloon an angioplasty-type dilation catheter is inserted through a compressed or not-fully expanded prosthesis until the balloon portion of the catheter is longitudinally aligned within the prosthesis. The balloon is then expanded forcing the prosthesis radially outwardly.

Once expanded the prosthesis remains in the expanded condition, and the strength of the prosthesis resists radial collapse. When used alone the prosthesis can expand and resist re-collapse of a previously collapsed or stenosed corporeal lumen. When used in combination with a graft, the prosthesis can maintain the graft open and secure the graft to the vessel.

Additional preferred embodiments of the present invention may provide benefits for high-expansion ratios. That is, the prosthesis may be configured to readily withstand high degrees of expansion and compression. Prostheses having loop or eyelet connectors according to this invention may also include beams of different lengths. Alternating pairs of beams having longer lengths and shorter lengths provide a more controlled expansion. This configuration also permits the eyelets of the shorter length beams to nestle below the eyelets of the longer length beams upon compression. A further feature aiding the expansion of the prosthesis includes varying the widths of the individual beams. For example, configuring beams having longer lengths with greater widths will improve the prosthesis ability to accomplish high expansion. Furthermore, varying the width along the length of the beam may also improve the expansion and compression abilities of prosthesis.

Other configurations of the prosthesis may be beneficial when the prosthesis is used in combination with a graft. The ends of prosthesis which are to be configured within a graft may include a flattened bulbous tail. Such an extension of the prosthesis prevents wear on the fabric of the graft. Eyelets on the ends of the prosthesis may also be used for stitching the prosthesis together with the graft. Eyelets provide a good anchoring point for such stitching. Various combinations of connected prosthesis according to the present invention may be used within grafts.

In another aspect, the invention relates to a prosthesis or stent having a plurality of cells. Each cell has a bottom end and a top end. A flattened bulbous tail is at the bottom end of at least more than one of the cells. A flattened bulbous tail may also be at the top end of at least more than one of the cells. Alternatively, the top end of each cell may include an apex having a smaller surface area than the flattened bulbous tail. To facilitate compression of the stent for delivery, adjacent flattened bulbous tails may be staggered longitudinally and the flattened bulbous tails may contour into the body of the stent. The circumference of the bottom and top ends of the stent may also include a rounded or chamferred edge.

In a further aspect, the invention relates to a method of manufacturing a stent from a flat member that includes a first surface and a second surface. Material is removed from the flat member such that the remaining material forms a pattern of a circular array of cells of a desired stent pattern. A mandrel having an outside surface with a shape is placed at the center of the pattern on the first surface of the flat member and the flat member is formed around the mandrel, thereby causing the flat member to assume a seamless tubular shape and becoming a tubular member.

The flat member may include a Nitinol sheet. The removal step may include chemical etching, laser cutting, electrical discharge machining, water-jet cutting, or stamping the stent pattern from the flat member. The outside surface of the mandrel may include a cylindrical shape having a diameter within the range of 20 mm to 34 mm. The forming step may include placing a collar on the second side of the flat member around the center of the pattern. The collar includes an inner surface and an outer surface with the inner surface having the same shape as the mandrel, but being larger than the outside surface of the mandrel. The size difference between the outside surface of the mandrel and the inside surface of the collar is about the same as the thickness of the flat member. The forming step may further include providing relative movement between the mandrel and the collar so that the collar causes the flat member to surround the outside surface of the mandrel, thereby causing the flat member to become the tubular member. The method may further include the step of heat treating the tubular member. The tubular member may be heat treated to a temperature of around 280° C. for a duration of around three minutes, then cooled immediately. The heat treating step may be performed while the tubular stent it is housed between the mandrel and the collar.

These and other advantages of the invention will become more apparent from the following detailed description of the preferred embodiments. When taken in conjunction with the accompanying exemplary drawings, the person of skill in the art will appreciate that various embodiments incorporate the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a first embodiment of the prosthesis in an expanded condition;

FIG. 2 is a perspective view of the first embodiment of the prosthesis in a compressed condition;

FIG. 3 is a cross-sectional view of the first embodiment of the prosthesis in an expanded condition;

FIG. 4 is a cross-sectional view of the first embodiment of the prosthesis in a compressed condition;

FIG. 5 is a perspective view of a second embodiment of the prosthesis;

FIG. 6 is a side view of a third embodiment of the prosthesis;

FIG. 7 is a top view of a portion of a flat pattern for the prosthesis;

FIG. 8 is a side view of a fourth embodiment of the prosthesis;

FIG. 9 is a side view of a vascular graft secured in a corporeal lumen by a prosthesis;

FIG. 10 is a side view of a prosthesis embedded in a corporeal lumen;

FIG. 11 is a perspective view of a thick-walled cylindrical tube with cells designed therein;

FIG. 12 is a flat pattern view of a portion of a prosthesis embodying variable thickness beams;

FIG. 13 is a flat pattern view of a portion of a prosthesis including alternative embodiments of variable thickness beams;

FIG. 14 is a flat pattern view of a portion of a prosthesis including additional alternative embodiments of variable thickness beams;

FIG. 15 is a flat pattern view of a portion of a prosthesis including additional alternative embodiments having varying flexibility;

FIGS. 16aand16bare side views of a first alternative embodiment of the beam ends and connector;

FIGS. 17aand17bare side views of a second alternative embodiment of the beam ends and connector;

FIG. 18 is a flat pattern view of a first embodiment of a prosthesis having the alternative beam end connectors shown inFIGS. 16aand16b;

FIG. 19 is a flat pattern view of a second embodiment of a prosthesis having the alternative beam end connectors shown inFIGS. 16aand16b;

FIG. 20 is a flat pattern view of the prosthesis ofFIG. 19 in a collapsed state;

FIG. 21 is a flat pattern view of a prosthesis from within a vascular graft;

FIG. 22ais a flat pattern view of a prosthesis having a flattened bulbous tail at a bottom end of each of the cells of the prosthesis;

FIG. 22bis a flat pattern view of a prosthesis having a flattened bulbous tail at the bottom end and the top end of each of the cells;

FIG. 22cis a cross-sectional view depicting contact points between the flattened bulbous tails of a prosthesis and a corporeal lumen wall;

FIG. 22dis a cross-sectional view depicting an end of the prosthesis ofFIG. 22ahaving a radius along an edge;

FIG. 22eis a cross-sectional view depicting an end of the prosthesis ofFIG. 22ahaving a chamfer along an edge;

FIG. 22fis a plan view depicting a tubular structure having atraumatic ends;

FIG. 23ais a perspective view of a flat pattern layout for a stent design;

FIG. 23bis a perspective view of the flat pattern layout of the stent ofFIG. 23apositioned with a mandrel and a collar prior to forming of the stent into a tubular shape;

FIG. 23cis an elevation view of the flat pattern layout of the stent ofFIG. 23apositioned with the mandrel and collar ofFIG. 23bprior to forming of the stent into a tubular shape; and

FIG. 23dis an elevation view of the stent ofFIG. 23aformed into the tubular shape and positioned between an interior surface of the collar and an exterior surface of the mandrel.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description, as well as the Figures, describe embodiments of the invention. These embodiments are exemplary of the inventors known uses of the invention, and are not intended to limit the scope of the claimed invention. Those skilled in the art of endoluminal devices will appreciate that the invention described herein may encompass many embodiments.

As shown in the Figures, the present invention relates to an endoluminal prosthesis. More particularly, the invention is an expandable and compressible prosthesis for repairing corporeal lumens. The prosthesis may be formed from a metallic cylinder by removal of cells. The invention also discloses a prosthesis with a flattened bulbous tail. The invention further discloses a stent being formed from a flat sheet of material.

As depicted inFIGS. 1 and 2, the result of removingcells20 from themetallic cylinder22 is aprosthesis24 having a series ofcurved beams26 and mergesections28. It is to be recognized that theprosthesis24 shown inFIG. 2 can be compressed, where desired, to a smaller diameter such that thecells20 are essentially defined by slits (not shown).

Thebeams26 are generally longitudinal members circumferentially spaced about theprosthesis24. In one embodiment, as depicted inFIG. 5, the ends of thesebeams26 merge with the ends of circumferentially adjacent beams to form themerge sections28 only at the ends of the prosthesis. The ends of eachbeam26 merge with one adjacent beam on theforward end30 and the opposite adjacent beam on therear end32. This creates a single circumferential pattern of staggered half-cells21 divided by beams26. (For comparison, afull cell25 is identified inFIG. 1). Preferably, thebeams26 each include at least twocurved segments34 of opposite orientations and aninflection point36 near the mid-point of the beams.

In the embodiment depicted inFIG. 5, themerge sections28 include either the forward ends30 orrear ends32 of twosuch beams26 as well as the ends of the prosthesis. These mergesections28 are also circumferentially spaced about theprosthesis24, preferably equidistantly.

The single pattern as depicted inFIG. 5 may be extended to buildlonger prostheses24. This may be done by extending the length of eachbeam26 with additionalcurved segments34 and formingadditional merge sections28. This forms theprosthesis24 as depicted inFIGS. 1 and 2. Alternatively, multiple single patterns may be connected with aseparate connector element58. Theconnector elements58 may have various configurations and be distributed throughout theprosthesis24 in a variety of arrangements. Such aprosthesis24, having “S”-shaped connector elements at each merge section is depicted inFIG. 6.

Other embodiments (not shown) may have connectors with other shapes only at every second or third merge section. Such embodiments may have advantages in providing longitudinal flexibility to theprosthesis24.

In the embodiments depicted inFIGS. 1 and 2 thebeams26 may extend beyond thefirst merge sections28 to formadditional merge sections28. This configuration may also be viewed as themerge sections28 connected end-to-end with opposite facingmerge sections28. This may provide for aprosthesis24 of greater lengths. Thecontinuous beams26 of this embodiment merge withadjacent beams26 repeatedly and alternately throughout their length. The continuous beams are comprised of multiplecurved segments34. Themerge sections28 may also contain flat segments (not shown).

As depicted inFIGS. 3 and 4 theprosthesis24 is preferably formed from a thick-walled cylinder22. The difference between the external radius of the cylinder and the internal radius of the cylinder defines aradial thickness40. Preferably, thecells20 of theprosthesis24 are removed such that the remainingbeams26 have a width (measured circumferentially)42 that is less than theradial thickness40. A typical design might have dimensions of 0.007″circumferential width42 and 0.014″radial thickness40. This defines a deep cross-section for thebeams26. To take advantage of the benefits of this invention, theradial thickness40 of thebeams26 needs to be substantially greater than thecircumferential width42. Preferably, theradial thickness40 will be at least one and one-third (1⅓) times thecircumferential width42.

A theoretical flat pattern of thebeams26 and mergesections28, as depicted inFIG. 7 reveals the novel configuration of the beams. Preferably, eachbeam26 is continuously curved, alternating betweencurves56 of opposite orientations throughout its length. In this ideal configuration, thebeams26form inflection points36 between the opposite facing curves56.

Eachcurve56 in eachbeam26 is defined by two radii, aninternal radius64 and anexternal radius66. The difference between these radii define thecircumferential width42 of thebeam26.

The continuously curved configuration ofbeams26 disposed longitudinally along a cylinder, provides some of the unique properties of this invention. As the cylinder is expanded from a partially compressedconfiguration50, the radii of eachcurve56 within eachbeam26 becomes smaller as the beams spread apart. Since thebeams26 are ideally continuously curved, the bending is spread throughout the entire length of thebeam26. This spreads the resultant stresses throughout thebeam26 and reduces or minimizes stress concentrations.

As depicted inFIGS. 2 and 4 the use of deep cross-sections has significant advantages for collapsing theprosthesis24 in preparation of intraluminal delivery. The deep cross-section allows for significant compression without incidental overlapping of thebeams26. Thelarge radial thickness40 of thebeams26 prevents one beam from extending over the top of another.

As depicted inFIGS. 1 and 3 there are also advantages to the use of deep cross-sections in expansion of theprosthesis24. In general, as a cylindrical, expandable prosthesis is expanded, longitudinally-oriented members of the collapsed prosthesis tend to bend circumferentially. The relatively narrow width of thebeams26 of thepresent prosthesis24 permits circumferential bending without inducing high stress concentrations. The large overall cross-sectional area of thebeams26 prevents re-compression of theprosthesis24. The configuration of thecurved segments34 spreads the stresses induced by expansion across the entire length of thebeams26, also reducing stress concentrations. In a preferred embodiment, the prosthesis is self-expandable. Alternatively, the prosthesis may be expanded by balloon.

FIGS. 1 and 3 andFIGS. 2 and 4 depict two separate configurations of theprosthesis24. Theprosthesis24 of the present invention has an expandedconfiguration44 while deployed in the lumen as depicted inFIGS. 1 and 3. This configuration has a large inner diameter which allows maximum patency of thelumen46 to be treated. Theprosthesis24 of the present invention also has a second, partially compressedconfiguration50 as depicted inFIGS. 2 and 4. This configuration is beneficial to the intraluminal delivery of the device which is facilitated by a smaller external diameter.

In a typical procedure, theprosthesis24 will be constrained in thecompressed configuration50 within a catheter. The catheter may then be inserted into asmall diameter lumen46, such as the femoral artery. To prevent damage to such an artery the entire system of catheter andprosthesis24 must have as small a diameter as possible. Small diameters also facilitate the navigation of theprosthesis24 and catheter through arduous vasculature. Once inserted into such an artery, the catheter andprosthesis24 may be advanced through the corporeal lumens, possibly to larger arteries for treatment. Theprosthesis24 may then be released from the catheter. Spring forces within the compressed prosthesis of a self-expanding version will force the prosthesis from the partially compressedconfiguration50 into the expandedconfiguration44. In a preferred embodiment, the spring forces are great enough to expand the lumen of the diseased vessel as theprosthesis24 expands. These forces are also great enough to impinge thebeams26 into the tissues of the vessel. This impinging secures theprosthesis24, and possibly an associatedgraft52, into place.

Another embodiment of theprosthesis24, depicted inFIG. 8, has a conical rather than cylindrical shape while in the expandedconfiguration44. In this embodiment, theprosthesis24 has a cylindrical shape in thecompressed configuration50. Upon expansion, however, abroader end60 of theprosthesis24 expands more than anarrower end62. This conical embodiment of theprosthesis24 is useful in similarly shaped lumens and various configurations of grafts. Thebroader end60 may includecells20 that are longer and wider in the expandedconfiguration44 than those at thenarrower end62.

Theprosthesis24 of the current invention may be used in a variety of procedures, two of which are depicted inFIGS. 9 and 10. As depicted in FIG.9, one or more prostheses embodying the present invention may be used in the treatment of aneurysms. An aneurysm is a weakening of the vessel wall of a vein or artery which causes a sack, or possibly a rupture, to form in thelumen46. When an aneurysm forms in the abdominal aorta, the condition can be life-threatening. A known treatment for aneurysms is the intraluminal delivery and implantation of avascular graft52. Such agraft52 bypasses the sack formed by the aneurysm and isolates the weakened tissues from the blood flow. To operate properly, thegraft52 must have leak-proof fixation to the healthy vascular tissue on either side of the aneurysm. Theprosthesis24 described herein may provide that fixation at one or more ends of thegraft52. Theprosthesis24 may also extend throughout the length of thegraft52. When expanded, theprosthesis24 may compress theflexible graft material52 against the arterial wall. Preferably, theprosthesis24 extends further from the aneurysm than thegraft52 so that parts of theprosthesis24 are imbedded in healthy tissue. This configuration maintains the patency of the artificial lumen of thegraft52 as well as securing the graft in place by forcing the end of the graft against the wall of thelumen46. Theprosthesis24 also ensures a leak-proof seal.

As depicted inFIG. 10, aprosthesis24 embodying the present invention may be used to treat a stenosis or collapse of thelumen46. Stenosis is often caused by the gradual occlusion of veins or arteries through the build-up of plaque. Preferably asingle prosthesis24 is inserted into the diseased vessel while mounted within a catheter. When theprosthesis24 is at the location of the narrowing, theprosthesis24 may be expanded. As depicted inFIG. 10, the spring forces of the prosthesis are preferably sufficient to expand the narrowed vessel. Theprosthesis24 is thereby forced into the tissues of thelumen46 to secure theprosthesis24 in place. The structure of theprosthesis24 resists collapse after expansion.

Theprosthesis24 may be manufactured in thecompressed configuration50, as inFIG. 2, or in the expandedconfiguration44, as inFIG. 1, or in any configuration in between. The manufacturing procedure requires the removal ofcells20 from a thick-walled cylinder22. This may be accomplished with several known manufacturing methods, such as laser cutting, chemical etching, photo-etching, electrical discharge machining (EDM) and mechanical means. Two materials found to be particularly suited to this application are implantable stainless steel and Nickel—Titanium alloys, such as Nitinol.

As depicted inFIG. 11, eachcell20 of theendoprosthesis24 may consist of twosides54 having threecurves56 and twoinflection points36. Such a configuration produces almond-shaped cells. There may also be flat portions (not shown) designed into thecell20. Thesecells20 are designed on the thickwalled cylinder22 in a pattern which repeats along the length of thecylinder22. This pattern is repeated with a longitudinal stagger of half acell20 around the circumference of thecylinder22. The pattern also includes half cells at each end of the tube. Upon removal of thecells20, the remaining material constitutes theprosthesis24 described herein.

Theprosthesis24 may be formed from a thickwalled cylinder22 approximately the size of the compressedconfiguration50. This thickwalled cylinder22 may be a Nickel Titanium alloy.Cells20 are laser cut into the thickwalled cylinder22 while the thickwalled cylinder22 is mounted over a wire. Thecells20 are formed in a long, narrow configuration with each of thecurves56 having large radii.

After thecells20 are cut into the thickwalled cylinder22, theprosthesis24 is cleaned and deburred to eliminate manufacturing irregularities. This may include blasting techniques, acid etching, ultrasonic cleaning and/or other well known methods of cleaning.

The prosthesis may then be stretched into more expanded configurations. One method of expanding the prosthesis is by mechanically stretching it over a mandrel. The mandrel may be specifically designed with pins to maintain the desired curvature of the beams. Once stretched, the prosthesis is annealed to set the new expanded shape of the prosthesis. Annealing can be accomplished by heating the prosthesis within a variety of media, such as air, molten salt, inert gas or vacuum. Annealing at 260–288° C. is appropriate for Nickel—Titanium alloys. After stretching, theprosthesis24 is cleaned again. This process of stretching, annealing and cleaning can be repeated until the desired configuration is obtained. Once the desired configuration has been obtained, the prosthesis is electropolished by any of the well-known methods.

Alternatively, aprosthesis24 may be formed from a Nickel Titanium thickwalled cylinder22 approximately the size of the expanded configuration. In this process,cells20 are cut into the thick walled cylinder in a shorter and wider configuration. This method would eliminate the need to stretch and anneal theprosthesis24 to achieve the expandedconfiguration94.

As best seen inFIGS. 12–14, it is also contemplated that the beams of a prosthesis may embody variable width beams or struts70 and generally uniform width beams or struts71. The incorporation of variable width struts70 into a prosthesis facilitates uniform expansion. For example, to achieve uniform expansion, it is desirable to havestruts70 of the same width meeting at connectingjunctions72.Asymmetric prosthesis portions74,76, as shown inFIGS. 12 and 13, may further require thestrut70 to embody a width that gradually varies along the length of thestrut70. Moreover, as shown inFIG. 14, where aprosthesis portion78 embodies a plurality of adjacent orientedcells80, the point ofconnection82,83 betweenadjacent cells80 may be varied in length, for example to accommodate ahole84. To facilitate uniform expansion of such aprosthesis portion78, thestruts70 extending from a relatively shorter point ofconnection82 betweenadjacent cells80 can embody a tapering thickness.

The novel features of the present invention may be applied to configure a prosthesis having variable properties throughout the length of the prosthesis. As an example, and as depicted inFIG. 15, the flexibility of theprosthesis24 may vary along the length of the prosthesis. To accomplish this,connector elements58 may be used to combinesegments90 of theprosthesis24. Eachsegment90 may be composed ofcurved beams26 in the various configurations described above. In a preferred embodiment, asegment90 composed offull cells20 may be combined withmultiple segments90 composed of half-cells21. The portion of theprosthesis24 composed of half-cell21segments90 will tend to be significantly more flexible longitudinally and slightly more flexible radially. The invention includes any combination of full cells and half-cells in a prosthesis including, but not limited to, full cells between half-cells.

One application of aprosthesis24 having variable flexibility throughout its length is for the support of avascular graft52. In such an application, the moreflexible segments90 of theprosthesis24 may be configured to support theartificial lumen46 of thegraft52. The lessflexible segments90 of theprosthesis24 may be configured to extend beyond theartificial lumen46 and into the patients natural lumen. In this manner, the lessflexible segments90 help secure thegraft52 into place, while the moreflexible segments90 support the material of thegraft52.

The invention described herein may also embody features to facilitate the high ratios of expansion possible with theprosthesis24. As depicted inFIGS. 16aand16b, as well as inFIGS. 17aand17b, the ends92 of thebeams26 may be connected in a manner which evenly distributes the stresses incurred by expansion and compression.

An eyelet or loop connector94 (shown inFIGS. 16aand16b) may connect the ends of thebeams26. Theseeyelet connectors94 distribute the stresses created by compression of theprosthesis94. As theprosthesis24 is initially compressed andadjacent beams26 are brought together, the bending and resultant stresses are initially concentrated in the eyelet or loop portion of theconnector94. Eventually, acontact area96 is formed at the edge of theeyelet connector24. As theprosthesis24 is further compressed andadjacent beams26 are brought even closer together, the bending and resulting stresses are concentrated at the ends of thebeams26 near thecontact area96. Even further compression may relieve the stresses in theeyelet connector24 by creating a fulcrum at thecontact area96.

Aprosthesis24 composed entirely ofeyelet connectors94, as depicted inFIG. 18, may facilitate the distribution of stresses induced by high expansion ratios. Thus, theprosthesis24 of the present invention may be used in particularly large corporeal lumens, such as the abdominal aorta. Thissame prosthesis24 may also be introduced into relatively small corporeal lumens, such as the femoral artery. Such an application requires the prosthesis to transition between a highly compressed state for insertion into the femoral artery, to a highly expanded state for implantation into the abdominal aorta. This application, as well as others, induce high stresses on theprosthesis24 through bending of thebeams26 in expansion and compression.

A similar distribution of the stresses may be accomplished by configuring theends92 of thebeams26 into increased contact end connectors98 (FIGS. 17aand17b). In such a configuration (depicted inFIGS. 17aand17b), the ends92 of thebeam26 connect together with a substantial area ofcontact96 near the actual connection. As theprosthesis24 is compressed and thebeams26 are brought closer together, the stresses due to bending are concentrated in thebeams26 near thecontact area96. Thecontact area96 expands as thebeams26 are brought closer together and the stress concentrations are thereby distributed along the length of thebeams26.

To further support high expansion ratios, theprosthesis24 of the present invention may be configured to pack tightly for compression into a collapsed state. One example, as depicted inFIGS. 19 and 20 may utilizeeyelet connectors94 aligned to differing heights. That is, everyother eyelet connector94 may be configured uponbeams26 of a first,greater length100, while eachother eyelet connector94 may be configured uponbeams26 of a second,lesser length102. Thus theeyelet connectors94 configured upon beams at the second length would have their greatest width at the same location that theeyelet connectors94 configured upon beams at the first length have their least width. In this manner, thebeams26 andeyelet connectors94 fit together in the most compact condition while compressed. Similar results may be accomplished by varying the thickness of thebeams26 andconnectors94.

Varying thecircumferential width42 of thebeams26 may also provide benefits in high expansion ratios. For example, using larger widths onbeams26 of a first,greater length100 may help control the expansion of theprosthesis24 and reduce stress concentrations. Varying thecircumferential width42 along the length ofindividual beams26 may provide superior nesting when the prosthesis is provided witheyelet connectors94.

Further configurations, as depicted inFIG. 21, may be advantageous when theprosthesis24 is configured for use in avascular graft52.Eyelet connectors94 may be used to provide an anchor for thestitching104 between thegraft52 and theprosthesis24. Themerge sections28 at the end of the prosthesis may include a flattenedbulbous tail106. Thetails106 reduce the wearing on the fabric of thegraft52.Tails106 may also help control the expansion of theprosthesis24. Instead of springing open when the ends of theprosthesis24 are released, thetails106 may remain constrained within a delivery catheter and provide theprosthesis24 with a slower, more controlled expansion.

As depicted inFIG. 22a, an alternative configuration of a prosthesis orstent120 for use in a vascular graft assembly includes a flattenedbulbous tail122 at a bottom124 end of at least more than onecell126 of the stent. In one embodiment, the bottom124 end of eachcell126 includes a flattenedbulbous tail122.Adjacent tails122 are staggered longitudinally, thus allowing each tail to attain a maximum size without interfering with compression of thestent120 for packing of the stent into a delivery catheter. The flattenedbulbous tails122 include larger surface areas than theapices128 at the top130 of thestent120.

While a vascular graft assembly typically includes at least one fixation stent that attaches a tubular graft to a corporeal lumen, the flattenedbulbous tail stent120 of the present invention aids in sealing the graft to the corporeal lumen. In one embodiment, the tubular graft of the vascular graft assembly includes a first end region and a second end region. Thestent120 may be located within the first end region of the graft with the flattenedbulbous tails122 of the bottom124 end of the stent positioned inside the tubular graft and theapices128 at the top130 end of the stent located beyond the first end region of the graft. Such placement of thestent120 positions theapices128 of the stent external to the graft.

Anytime a stent or prosthesis is deployed in a graft, there is an opportunity for wear. The flattenedbulbous tail122 of thestent120 reduces the amount of wear between the stent and the softer material of the vascular grafts. Prosthesis or stent to graft wear is a function of the radial force per unit area of the stent, the sharpness of the stent, and the amount of relative motion between the stent and the graft. In the present invention, the relatively large surface area of the flattenedbulbous tail122 displaces the radial force of the tail over a larger area, thereby effectively reducing the radial force per unit area of the stent, and the sharpness of thestent120 is decreased by the circular shape of the flattened bulbous tail. By reducing the radial force per unit area and decreasing the sharpness through the use of the flattened bulbous tail, the amount of graft wear for a given stent over a given time is reduced, thereby prolonging the useful life of the graft. The flattenedbulbous tail122 configuration disclosed herein may also be applied to the previously disclosed prostheses and stents.

Referring toFIG. 22b, flattenedbulbous tails122 may replace the apices at the top130 end of at least more than onecell126 of thestent120 or prosthesis in order to minimize wear at the contact point between a stent and a corporeal lumen wall132 (FIG. 22c). In one embodiment, the top130 end of each of thecells126 and the bottom124 end of each of the cells within thestent120 includes a flattenedbulbous tail122.FIG. 22cdepicts the contact points between one embodiment of astent120 having flattenedbulbous tails122 and thecorporeal lumen wall132.FIG. 22bdepicts the flattenedbulbous tails122 at the bottom124 end and top130 end of thecells126 offset longitudinally such that only one half to the flattened bulbous tails occupy the same axial location when thestent120 is compressed for delivery.

An alternate method of minimizing wear at the contact point between astent120 and thecorporeal lumen wall132 or graft material is to grind a radius134 (FIG. 22d) or chamfer136 (FIG. 22e) around the circumference of the bottom124 end and the top130 end of each of thecells126. Alternatively, if thestent120 is laser cut from a Nitinol tube138 (FIG. 22f), the bottom124 end and top130 end of the tube may be ground prior to cutting thecell126 pattern within the stent. With aradius134 ground around the circumference of the bottom124 or top130 end of each of thecells126, the contact point between thestent120 and thevessel lumen wall132 or graft material is the tangent point of the radius.

A method for fabricating stents from a flat sheet of material is depicted inFIGS. 23a–23d. The general geometry of the stent pattern140 (FIG. 23a) can be cut directly from aworkpiece142 of a substantially flat piece of material. In a preferred embodiment, theworkpiece142 includes a shape memory sheet metal material, such as a Nitinol sheet. Cutting the general geometry of the stent pattern may be accomplished by any of several known manufacturing methods, such as laser cutting, chemical etching, photo-etching, electric-discharge machining (EDM), water-jet cutting, stamping, and other mechanical means.

Material removal, such as the cutting of thestent pattern140, is easier to perform on a flat piece of material than on a tubular piece of material. For instance, when cutting a stent pattern from a tubular piece of material, it is often necessary to place a secondary filler material into the lumen of the tube to prevent damage to one side of the tube while an opposite side of the tube is being cut. Also, it is often necessary to rotate a tubular piece of material about its axis while a stent pattern is being cut into it. While these steps may add to the complexity of cutting a stent pattern into a piece of material, they may also add to the cost to cut the stent pattern. Further, the cost of raw tubular material is often higher than the cost of sheet material. Therefore, producing a tubular stent from a flat sheet of material may reduce the cost of producing the stent.

Thestent pattern140 may include a circular array ofcells143. After thestent pattern140 is produced, acylindrical mandrel144 may be placed at thecenter146 of thepattern140 within theworkpiece142 from a first side of the flat sheet and acollar148 may be placed around the center of the pattern on a second side of the flat sheet. Relative movement of thecollar148 and themandrel144 toward each other causes theflat workpiece142 to form around the mandrel and to assume a seamless tubular shape150 (FIG. 23d) between the mandrel and the collar. Although a cylindrical-shape mandrel144 andcollar148 are disclosed, other shapes may also be used, such as a conical shape.

In a preferred embodiment, the outer diameter of themandrel144 ranges from 20 mm to 34 mm, depending on the size of thestent152 to be produced, however a smaller or larger diameter can be used. Themandrel144 can also incorporate grooves (not shown) or pins (not shown) which coincide with thecells143 to control or change the shape of thestent152. Theinside surface154 of thecollar148 may have the same shape as theoutside surface156 of themandrel144, but with the collar in place over the mandrel there is agap160 all around about the same size as the thickness of thestent152.

Thestent152 may be heat treated, such as in a furnace (not shown) or in a salt pot (not shown), while housed between themandrel144 and thecollar148. During the heat treatment process, the material may be heated to a temperature of around 280° C. (535° F.) for a duration of about three minutes, and then cooled rapidly to set the shape into the material. The method disclosed herein for fabricating stents from a flat sheet of material may also be applied to the previously disclosed prostheses and stents.

While the present invention has been described herein in terms of a prosthesis or stent for the repair of blood vessels, those of skill in the art will readily recognize that prostheses embodying the described invention can be used to treat a variety of corporeal lumens, for example the bronchial tree or intestines. The invention described herein is intended to be limited only by the claims that follow and not by any particular embodiment.

Claims (10)

1. A prosthesis, comprising:

a plurality of cells defined by struts having a first width, each cell having a bottom end and a top end, the struts defining a surface exterior of the cell;

a flattened bulbous tail attached to the surface exterior of the cell adjacent the bottom end of at least more than one of the cells; and

a flattened bulbous tail attached to the surface exterior of the cell adjacent the top end of at least more than one of the cells;

wherein the flattened bulbous tails include a perimeter, a tail surface extending across a lateral face of the tail surface bounded by the perimeter, the lateral surface having a second width greater than the first width.

2. The prosthesis ofclaim 1, wherein the bottom end of each cell includes a flattened bulbous tail.

3. The prosthesis ofclaim 1, wherein the top end of each cell includes a flattened bulbous tail.

4. The prosthesis ofclaim 1, the flattened bulbous tails at the bottom end of the cells contouring into the body of the prosthesis and the flattened bulbous tails at the top end of the cells contouring into the body of the prosthesis.

5. The prosthesis ofclaim 1, wherein:

adjacent flattened bulbous tails at the bottom end of the cells are staggered longitudinally; and

adjacent flattened bulbous tails at the top end of the cells are staggered longitudinally.

6. The prosthesis ofclaim 1, further comprising:

a substantially rounded edge about the circumference of the prosthesis at the bottom end; and

a substantially rounded edge about the circumference of the prosthesis at the top end.

7. The prosthesis ofclaim 1, further comprising:

a substantially chamferred edge about the circumference of the prosthesis at the bottom end; and

a substantially chamferred edge about the circumference of the prosthesis at the top end.

8. A prosthesis, comprising:

a plurality of cells defined by struts having a first width, each cell having a bottom end and a top end, the struts defining a surface exterior the cell;

a flattened bulbous tail attached to the surface exterior the cell adjacent the bottom end of each cell, wherein adjacent flattened bulbous tails are staggered longitudinally and each flattened bulbous tail contours into the body of the prosthesis; and

a flattened bulbous tail attached to the surface exterior the cell adjacent the top end of each cell, wherein adjacent flattened bulbous tails are staggered longitudinally and each flattened bulbous tail contours into the body of the prosthesis;

wherein the flattened bulbous tails embody a continuous surface that has a second width greater than the first width.

9. The prosthesis ofclaim 8, further comprising:

a substantially rounded edge about the circumference of the prosthesis at the bottom end; and

a substantially rounded edge about the circumference of the prosthesis at the top end.

10. The prosthesis ofclaim 8, further comprising:

a substantially chamferred edge about the circumference of the prosthesis at the bottom end; and

a substantially chamferred edge about the circumference of the prosthesis at the top end.

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